Image input unit and image input method

ABSTRACT

The invention is characterized by the fact that an area sensor for outputting an analog signal responsive to the light reception amount of light of CMYG is used and when CMYG image data is converted into RGB image data, RGB image data having a domain also in negative values is generated without performing processing of putting the RGB image data into values of 0 or more, without decreasing the information amount of the CMYG image data. Further, the invention is characterized by the fact that when the image data is recorded in a file section  16  finally as a JPEG file, the pixel data of each color of YCbCr is represented as data type of eight bits and the information amount per pixel is 24 bits for recording more color information without increasing the memory capacity.

This is a continuation of application Ser. No. 10/618,690 filed Jul. 15,2003 now U.S. Pat. No. 7,042,465 which is a continuation-in-part ofApplication No. PCT/JP02/00478 filed Jan. 23, 2002. The entiredisclosures of the prior applications, application Ser. Nos. 10/618,690and PCT/JP02/00478 are considered part of the disclosures of theaccompanying continuation application and is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to an image input unit and an image inputmethod for inputting a color image to be printed out.

Hitherto, in a printer, complementary color ink has been used to finallyoutput image data representing the values of CMY (Cyan, Magenta, Yellow)or CMYK (Cyan, Magenta, Yellow, black) and in a monitor, image datarepresenting the value of RGB (Red, Green Blue) is finally output.Generally, in image processing using a computer, color matching isperformed with the display colors of a monitor as the reference andtherefore in an image input unit such as a digital still camera, imagedata is represented in the area of RGB color space with the RGB valuestaking 0 or more. For example, in not only a digital still camera usingprimary color CCD, but also a digital still camera using complementarycolor CCD, when CMYG (Cyan, Magenta, Yellow, Green) image data providedfrom an input image is converted into RGB image data, processing ofadjusting the components of a conversion matrix or replacing negativevalues with 0 is performed and only RGB values of 0 or more are output.

For example, in sRGB standard, color coordinates of RGB are determinedand RGB color space wherein the RGB values are 0 or more is defined.However, the sRGB color space is narrower than the color space that canbe recognized by the visual sense of a human being and thus only a colorspace in a narrower range than the visual sense of a human being can berepresented in an image input unit for outputting an image representedin the sRGB color space.

On the other hand, in a printer, a color space wherein the CMY or CMYKvalues are 0 or more can be represented by printing. A printer foroutputting an image represented in the color space wherein the CMY orCMYK values are 0 or more can use pigments for color development on theoutside of the sRGB color space as CMY ink, thereby representing a widercolor space than the sRGB-defined color space by printing. Likewise, theprinter can represent a wider color space by printing than a colormonitor manufactured with sRGB display as the reference.

For such a printer to print image data recorded in an image input unitsuch as a digital still camera for recording an image represented in acolor space wherein RGB colors represented in sRGB are 0 or more, theimage is represented only in a narrower range than the colorrepresentation area of the printer, and the characteristics of theprinter are not used effectively; this is a problem.

In recent years, image data format standards allowing negative values inthe colors of sRGB64, RIMM-RGB, etc., have been developed. However, ifan attempt is made to allow negative values and improve the imagequality, the information amount per pixel is increased three bits andthus the image data formats result in an increase in the data capacity.

SUMMARY OF THE INVENTION

The invention is embodied considering such problems and it is an objectof the invention to provide an image input unit and an image inputmethod for expanding color space of a print image without increasing thedata capacity.

It is another object of the invention to provide an image input unit andan image input method for expanding color space of a print image withoutimpairing the compatibility with an image file format in a digitalcamera in a related art.

In order to solve the aforesaid object, the invention is characterizedby having the following arrangement.

-   (1) An image input unit comprising:

a data generator which generates RGB image data representing each valueof RGB for each pixel as signed data type from an input image; and

a data converter which converts the RGB image data into YCbCr image datarepresenting each value of YCbCr for each pixel.

-   (2) The image input unit according to (1), wherein the data    generator generates CMY or CMYG image data representing each value    of CMY or CMYG for each pixel as unsigned data type from the input    image, and converts the CMY or CMYG image data into the RGB image    data.-   (3) The image input unit according to (2), wherein the data    generator includes a signed arithmetic circuit for converting the    CMY or CMYG image data into the RGB image data.-   (4) The image input unit according to (2), wherein the data    generator generates CMY or CMYG image data representing each value    of CMY or CMYG for each pixel as unsigned data type of 10 bits or    more from the input image.-   (5) The image input unit according to (1), wherein

the RGB image data representing each value of RGB for each pixelgenerated by the data generator is signed data type of nine bits ormore, and

the RGB image data into YCbCr image data representing each value ofYCbCr for each pixel converted by the data converter is unsigned datatype of eight bits.

-   (6) The image input unit according to (1), wherein the data    converter enlarges color saturation represented by CbCr.-   (7) An image input method comprising:

a data generation step of generating RGB image data representing eachvalue of RGB for each pixel as signed data type from an input image; and

a data conversion step of converting the RGB image data into YCbCr imagedata representing each value of YCbCr for each pixel.

-   (8) The image input method according to (7), wherein at the data    generation step, CMY or CMYG image data representing each value of    CMY or CMYG for each pixel as unsigned data type is generated from    the input image, and the CMY or CMYG image data is converted into    the RGB image data.-   (9) The image input method according to (8), wherein at the data    generation step, CMY or CMYG image data representing each value of    CMY or CMYG for each pixel as unsigned data type of 10 bits or more    is generated from the input image.-   (10) The image input method according to (7), wherein

RGB image data representing each value of RGB for each pixel generatedat the data generation step is signed data type of nine bits or more isgenerated, and

YCbCr image data representing each value of YCbCr for each pixelconverted from the RGB image data at the data conversion step isunsigned data type of eight bits.

-   (11) The image input method according to (7), wherein at the data    conversion step, color saturation represented by CbCr is enlarged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a data flowchart to show the contents of data transferredbetween blocks shown in FIG. 2.

FIG. 2 is a block diagram to show a digital still camera 1 as oneembodiment of an image input unit according to the invention.

FIG. 3 is a schematic drawing to show an area sensor of the digitalstill camera according to the embodiment of the invention.

FIG. 4 is a schematic drawing to make a comparison between the area ofRGB color space and that of color space that can be recognized by ahuman being.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the invention will be discussed with reference withthe accompanying drawings.

FIG. 2 is a block diagram to show a digital still camera 1 as oneembodiment of an image input unit according to the invention. FIG. 1 isa data flowchart to show the contents of data transferred between theblocks shown in FIG. 2. An optical system 10, an area sensor 11, an AFE(Analog Front End) 12, and an image processing section 13 corresponds toa data generator in Claims, and a color space conversion section 14corresponds to a data converter in Claims.

The optical system 10 is constituted by an optical lens, an infrared cutfilter, an optical low-pass filter, etc., for forming a subject image asan input image on the area sensor 11.

The area sensor 11 is an optical sensor of CCD, a CMOS sensor, etc.,comprising photoelectric conversion elements for converting light intoan electric signal, and each photoelectric conversion element isprovided with a complementary color filter for allowing light ofwavelength of any of C (Cyan), M (Magenta), Y (Yellow), or G (Green) topass through. The photoelectric conversion elements are placed like amatrix as shown in FIG. 3. The description to follow assumes that thearea sensor comprising the photoelectric conversion elements eachprovided with a complementary color filter of any of C, M, Y, or G isused, but complementary color filters of three colors of CMY may be usedas the filters of the photoelectric conversion elements. The purpose ofacquiring color information of G in addition to color information of CMYis to directly acquire color information of G recognized sensitively bythe visual sense, thereby improving the image quality. The area sensor11 outputs an analog signal responsive to the light reception amount oflight of wavelength of any of CMYG in each photoelectric conversionelement to the AFE 12.

The AFE 12 is constituted by a program gain amplifier, a CDS circuit, anA/D converter, etc., for sampling the analog signal output from eachphotoelectric conversion element and generating a digital signalrepresenting the value of any of CMYG in each photoelectric conversionelement as 10 to 12-bit data type. The analog signal is quantized to adigital signal of data type having a length of 10 bits or more, wherebyhigh-quality YCbCr image data can be generated in the color spaceconversion section 14. The digital signal of CMYG is input to the imageprocessing section 13 directly or after it is stored in buffer memory.

The image processing section 13 is implemented as an ASIC or DSP(Digital Signal Processor) engine implementing a predetermined algorithmas a logical circuit. If the image processing section 13 is implementedas an ASIC, processing can be speeded up as compared with the case whereprocessing is performed by execution of a program in a DSP engine. Theimage processing section 13 performs AE (Auto Exposure), AWB (Auto WhiteBalance), image generation processing, conversion processing from CMYGcolor space to RGB color space, γ correction processing, etc. The imagegeneration processing mentioned here is mainly processing of generatingCMYG image data having four values of CMYG for each pixel by performinginterpolation processing, etc., using a digital signal representing thevalue of any of CMYG in each photoelectric conversion element. Theconversion processing from CMYG color space to RGB color space isperformed by a 4×3 matrix arithmetic processing circuit of ASIC or amultiplication circuit and an addition-subtraction circuit of DSP, CPU19, etc. To allow negative values as the values of RGB output as aresult of the conversion processing, the matrix arithmetic processingcircuit, the multiplication circuit, the addition-subtraction circuit,and the like used for the conversion processing are formed as signedarithmetic circuits. The RGB image data representing each value of RGBis represented as data type having a larger number of bits than thenumber of bits of CMYG image data by one bit corresponding to a signbit. For example, if CMYG image data is data type of a length of 12 bitshaving a domain of each color ranging from 0 to 2¹²−1, RGB image databecomes signed data type of a length of 13 bits having a domain of eachcolor ranging from −2¹²−1 to +2¹²−1. It is desirable that data type of alength of each color nine bits or more should be assigned to RGB imagedata for representing the data at least in the range of −64 to 255.Further, if the domain is expanded to the range of −160 to 255, an imageparticularly good in color reproducibility can be provided. Taking thedomain of R large in both positive and negative ranges leadsparticularly to improvement of the image quality.

The color space conversion section 14 is constituted by a 3×3 matrixarithmetic processing circuit or a multiplication circuit and anaddition-subtraction circuit of DSP, CPU 19, etc. The color spaceconversion section 14 performs linear conversion using a 3×3 matrix,thereby generating YCbCr image data representing the values of YCbCrfrom RGB image data. The matrix arithmetic processing circuit, themultiplication circuit, the addition-subtraction circuit, and the likeused for the conversion processing are formed as signed arithmeticcircuits. To compress and record image data in removable memory in aJPEG file format, the color space conversion section 14 needs to outputYCbCr image data rounded to eight bits. For the color space conversion,using the following expressions conforming to ITU-R BT.601Y=0.299R+0.587G+0.114BCb=(−0.299R−0.587G+0.886B)×0.564+offsetCr=(0.701R−0.587G−0.114B)×0.713+offsetfor example, the conversion expressionsY=0.2990R+0.5870G+0.1140BCb=−0.1687R−0.3313G+0.5000B+128Cr=0.5000R−0.4187G−0.0813B+128can be used.

In the conversion processing using the expressions, arithmeticprocessing is performed internally with precision of nine bits or morewith a sign and at the final output time, YCbCr is rounded to precisionof eight bits and each value of YCbCr is output as unsigned data type ofthe length of eight bits having a domain ranging from 0 to 255. Thepurpose of rounding YCbCr to precision of eight bits at the final outputtime is to make the image data compatible with JPEG compressionprocessing. In the process of the conversion processing, the values ofCbCr may be increased in an equal ratio for enlarging the sizes ofvectors represented by the values of CbCr, thereby increasing thebrightness of an image. When color saturation is enlarged, the colorsaturation is made large in response to the color saturation beforeenlargement so as to enlarge the distribution area of the colorsaturation of each pixel. For example, the color saturation beforeenlargement is multiplied by a constant, thereby making the colorsaturation large. Such processing results in diametrically enlarging aMunsell color solid that can be represented by the values of YCbCr. Suchprocessing is fitted particularly for simulatedly compensating forbrightness information lost in the process of performing arithmeticprocessing of the values of RGB.

A compression processing section 15 may be implemented as an ASICdedicated to compression processing or may be implemented as a DSP, CPU19, etc. The compression processing section 15 performs compressionprocessing conforming to the JPEG standard using DCT (Discrete CosineTransform) and Huffman coding, and outputs a JPEG bit stream to a filesection 16.

The file section 16 records image data compressed in the JPEG fileformat in removable memory such as flash memory. The image file formatneed not be JPEG and if the image file format is an image file formatconsisting of information of YCbCr, the invention can be applied; forexample, the image data may be recorded in the image file format ofTIFF-YCbCr.

A control section 17 includes: ROM 20 recording a control program, etc.;a CPU 19 for executing the control program, thereby controlling theoptical system 10, the area sensor 11, the AFE 12, and ASIC or DSPimplementing the image processing section 13, the color space conversionsection 14, and the compression processing section 15; and RAM 18 forstoring various pieces of data as the control program is executed. TheROM 20, the CPU 19, and the RAM 18 are connected each other by a bus.

The embodiment is characterized by the fact that the area sensor 11 foroutputting an analog signal responsive to the light reception amount oflight of wavelength of any of CMYG for each photoelectric conversionelement is used to generate CMYG image data and when the CMYG image datais converted into RGB image data, RGB image data having a domain also innegative values is generated without performing processing of puttingthe RGB image data into values of 0 or more, namely, without decreasingthe information amount of the CMYG image data. FIG. 4 schematicallyshows the color space of RGB taking values of 0 or more and the colorspace that can be recognized by a human being. As shown in FIG. 4, thecolor space that can be recognized by a human being is wider than thecolor space of RGB taking values of 0 or more. Thus, if conversion isexecuted from the color space of CMY taking values of 0 or more to thecolor space of RGB taking values of 0 or more as in the related art,loss of color information that can be recognized by a human beingoccurs, resulting in color distortion before and after the conversion.In the above-described embodiment, negative values are allowed as inputof the color space conversion section 14, so that the color space thatcan be represented by an image consisting of color information of RGB isexpanded and, for example, when an image is printed out usingcomplementary color ink, it is made possible to exhaustively representthe color information provided at the input time.

Further, the embodiment is characterized by the fact that to record theimage data in the file section 16 finally as a JPEG file, the YCbCrimage data is represented as unsigned data type of eight bits and theinformation amount per pixel just before compression processing isperformed is 24 bits for recording more color information withoutincreasing the memory capacity. This uses the nature such that even ifthe RGB image data contains a negative value, if the RGB image data isconverted into YCbCr image data according to the above-describedconversion expressions, the YCbCr image data is put in a value of 0 ormore.

That is, according to the embodiment, negative values are allowed forRGB image data and image processing is performed with precision of ninebits or more for each color of RGB until just before conversion to YCbCrcolor space, and after conversion to YCbCr color space, the data isrounded to the precision of each color eight bits, whereby the imagedata can be recorded without increasing the data capacity. Therefore, onthe output side, the values of RGB are output as data type having adomain to negative values at the inverse conversion time from the colorspace represented by YCbCr to the RGB color space, and conversion isexecuted from the RGB color space to the CMY color space without losingthe color information provided at the input time, so that color matchingbetween the input image of the digital still camera 1 and the printimage can be improved.

In the description of the embodiment, an input image is provided throughthe area sensor 11 comprising complementary color filters. However, ifthe area sensor 11 uses primary color filters of RGB, the imageprocessing section 13 generates RGB image data provided with negativevalues by performing matrix arithmetic processing from RGB image datahaving a value of 0 or more in each color, whereby it is also possibleto widen the area of the color space provided after γ correctionprocessing, for example.

In the description of the embodiment, the digital still camera isillustrated as the image input unit, but the invention can also beapplied to other image input units of a digital video camera, a scanner,a facsimile, etc.

1. An image input unit comprising: a data generator which generates RGBimage data representing each value of RGB for each pixel as signed datatype from an input image; and a data converter which converts the RGBimage data into YCbCr image data representing each value of YCbCr foreach pixel, wherein, the RGB image data representing each value of RGBfor each pixel generated by the data generator is signed data type ofnine bits or more, and the RGB image data into YCbCr image datarepresenting each value of YCbCr for each pixel converted by the dataconverter is unsigned data type of eight bits.
 2. The image input unitaccording to claim 1, wherein the data converter enlarges colorsaturation represented by CbCr.
 3. An image input method for inputing animage to be printed out, the method comprising: generating RGB imagedata representing each value of RGB for each pixel as signed data typefrom an input image; and converting the RGB image data into YCbCr imagedata representing each value of YCbCr for each pixel, wherein RGB imagedata representing each value of RGB for each pixel generated is signeddata type of nine bits or more, and YCbCr image data representing eachvalue YCbCr for each pixel converted from the RGB image data is unsigneddata type of eight bits.
 4. The image input method according to claim 3,wherein, during the data conversion, color saturation represented byCbCr is enlarged.